title: Ammonium secretion by Malpighian tubules of Drosophila melanogaster: application of a novel ammonium-selective microelectrode.
abstract: Ammonia is a toxic nitrogenous waste product of amino acid metabolism that may accumulate to high levels in the medium ingested by larvae of the fruit fly Drosophila melanogaster. Here we report measurements of haemolymph NH4(+) concentration and the secretion of NH4(+) by the Malpighian (renal) tubules. Measurement of NH4(+) concentrations in secreted droplets is complicated either by the requirement for large sample volumes for enzymatic assays or by the inadequate selectivity of NH4(+)-selective microelectrodes based on nonactin. We have developed a novel liquid membrane NH4(+)-selective microelectrode based on a 19-membered crown compound (TD19C6), which has been used previously in ammonium-selective macroelectrodes. In conjunction with an improved technique for correcting for interference of potassium, NH4(+)-selective microelectrodes based on TD19C6 permit accurate measurement of ammonium concentration in haemolymph samples and nanolitre droplets of fluid secreted by the Malpighian tubules of D. melanogaster. The results indicate that active secretion of ammonium into the Malpighian tubule lumen is sufficient to maintain concentrations of ~1 mmol l(-1) ammonium in the haemolymph of larvae reared on diets containing 100 mmol l(-1) ammonium chloride.
authors.affiliation: Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON, Canada, L8S 4K1.
authors.forename: Austin
authors.initials: A
authors.lastname: Browne
authors.forename: Michael J
authors.initials: MJ
authors.lastname: O'Donnell
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001826:Body Fluids / Q000187:drug effects / Q000378:metabolism; D004032:Diet; D004331:Drosophila melanogaster / Q000187:drug effects / Q000378:metabolism*; D006458:Hemolymph / Q000187:drug effects / Q000378:metabolism; D017736:Ion-Selective Electrodes; D007814:Larva / Q000187:drug effects / Q000378:metabolism; D008317:Malpighian Tubules / Q000187:drug effects / Q000378:metabolism*; D008839:Microelectrodes; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology; D012996:Solutions
chemical_list: D064751:Ammonium Compounds; D012996:Solutions; D012965:Sodium Chloride; D012964:Sodium; D011188:Potassium
keywords: Malpighian tubule; ammonium; haemolymph; ion-selective microelectrode